Earth structures

Abstract: An earth structure comprises a plurality of elongate stabilising elements (3) in an earth mass behind a facing made up of facing panels (1), and a plurality of connectors (2) behind the facing and connecting it to the stabilising elements (3). Each connector (2) has a rear attachment portion (8) attached to a respective stabilising element (3) and at least two spaced apart front attachment portions (6) attached to the facing. The earth mass comprises a first region (20) of soil suitable for plant growth, in which first region (20) the connectors (2) are located, and a second region (21) of structural backfill, in which second region (21) the stabilising elements (3) are located.

Description: The invention relates to earth structures, certain components for use in earth structures and to methods of constructing earth structures. It is known from European Patent Application No. 0 318 243 to provide an earth structure frictionally stabilised by a plurality of elongate stabilising elements in the form of strips extending rearwardly from a facing of the structure into an earth mass. The earth is stabilised throughout the mass by frictional engagement with the strips, thereby enabling the earth mass to behave as an elastic material with greatly improved resistance to failure. The facing of the known structure consists of a series of rows of "C" shaped mesh facing panels arranged one above another. The panels in each row are supported by laterally spaced support straps. These are also "C" shaped, each having an upright front portion in front of the panels and relatively short upper and lower rearwardly extending portions. These upper and lower portions are connected to an earth stabilising strip. Thus the forward end of each stabilising strip is located between a rearwardly extending upper portion at the top of a support strap in one row and a rearwardly extending lower portion at the bottom of a support strap in the row above. A bolt passes through the upper and lower rearwardly extending portions and the stabilising strip to form a secure connection. The advantages of using facing panels formed of mesh are that they are lightweight and thus inexpensive compared to eg. concrete panels and that they allow the growth of vegetation on the facing, thus giving it a "green" appearance. However, because of their lightweight nature, the mesh facing panels are flexible and thus subject to deformation. In particular, there is a tendency for the facing panels to bulge out where they span between the laterally spaced support straps. If it were desired, for aesthetic or other reasons, not to use the support straps and to connect the stabilising strips directly to the mesh facing panels, there would be an increased tendency for the panels to deform. The earth mass behind the facing may be structural backfill selected in a known manner to co-operate with the stabilising elements to produce a stable structure. The entire earth mass behind the facing may consist of such structural backfill. It is known from US-A-4 117 686 (on which the precharacterising part of claim 1 is based) to provided a stabilised earth structure comprising a plurality of elongate stabilising elements in an earth mass behind a facing, the earth mass comprising a first region of earth of a first type adjacent to the facing, and a second region of earth of a second type behind the first region. The present invention is characterised in that the first region of earth comprises soil suitable for plant growth and in that the earth structure further comprises a plurality of connectors behind the facing and connecting it to the stabilising elements, the connectors being located in said first region of earth and the stabilising elements being located in said second region of earth, whereby exposure of the stabilising elements to the first type of earth in the first region is substantially avoided. The first earth type is a type of soil, such as top soil, suitable for establishing plant growth to produce a "green" facing. The second earth type may be structural backfill. Earth of a type suitable for plant growth will generally contain organic matter and possibly fertilizers and will tend to have a high moisture content. This produces good conditions for plant growth but aggressive conditions for the earth stabilizing elements. However, by using connectors which are located in the first region of earth, the stabilising elements, which are located in the second region of earth, are not exposed to the aggressive conditions. It is thus possible to use conventional stabilising elements. On the other hand, the connectors can be designed with dimensions, the material they are made from and/or protective measures which take account of the aggressive conditions. For example, the connector may have a thickness which is greater than that which is structurally needed. Thus, where a connector made from a 10mm diameter bar would support the load, a 14mm bar may be used. This is an over thickness of 4mm, as compared to a 1mm over thickness which is typically used for a metal stabilising strip for a 70 year service life. Possible protective measures for the connector are galvanising or other metallic coating, e.g. zinc-aluminium alloy, applied by spraying or dipping. Plastic coatings, such as polyamide, polyurethane or epoxy, may also be used. Whilst the facing is preferably a mesh facing, there are other possibilities, such as a facing with openings through which plants can grow, for example being made up of concrete elements. In a preferred structure, the first and second regions of earth are separated by geosynthetic material, such as a sheet or sheets of geotextile. This helps to ensure that the stabilising elements are not exposed to the first earth region and also, by providing a clear boundary between the two earth regions, helps to ensure that the first region is of the correct thickness. The geotextile is preferably a non-woven product with good filtration and drainage properties. The provision of first and second earth regions is useful if the connectors are attached to the facing at only one front attachment point, as well as when there are at least two front attachment portions. A preferred embodiment comprises an earth structure comprising a plurality of elongate stabilising elements in an earth mass behind a mesh facing, and a plurality of connectors behind the facing and connecting it to the stabilising elements, each connector having a rear attachment portion attached to a respective earth stabilising element, and having at least two spaced apart front attachment portions attached to the mesh facing. It will be appreciated that the forward earth pressure on the mesh facing is withstood by the stabilising elements connected to the facing via the connectors. By attaching each connector to the mesh facing by the spaced apart front attachment portions, the load on the mesh facing applied by the connector is distributed between those attachment portions, thereby reducing the deflection of the facing. In facet, the connector arrangement may be useful with other types of facing where it is desired to limit the deflections by distributing the load thereon. Thus, a preferred embodiment comprises an earth structure comprising a plurality of elongate stabilising elements in an earth mass behind a facing, and a plurality of connectors behind the facing and connecting it to the stabilising elements, each connector having a rear attachment portion attached to a respective earth stabilising element, and having at least two spaced apart front attachment portions attached to the facing. For example, the facing may be made of a sheet or sheets of eg. metal. A preferred connector, for connecting an earth stabilising element and a facing, comprises a rear attachment portion for attachment to an earth stabilising element, and at least two spaced apart front attachment portions for attachment to a facing. The front attachment portions may take any convenient form but are preferably arranged to hook on to a bar or lug of the facing. Thus each front attachment portion may be in the form of a hook. In a preferred embodiment, the connector has two front attachment portions and is substantially "V" shaped. The front attachment portions are preferably spaced apart in a horizontal or lateral direction. The connector may be formed by bending a bar, for example a 14mm diameter steel bar. The connectors are preferably capable of pivoting about a horizontal axis at the facing. This can advantageously permit the connectors to be at an appropriate orientation, normally horizontal, for any angle of facing. In general, the slope of the facing can vary between 45° to the horizontal and vertical (90° to the horizontal). Pivotability of the connectors can advantageously be achieved by the hooks described above, which can pass round at least one substantially horizontal bar of the facing. The connectors extend rearwardly into the earth so as to have a length in this direction which is substantially less than the length of the stabilising elements, for example less than one quarter, preferably less than one fifth. In a preferred embodiment, a mesh facing comprises mesh panels arranged one above another, and the connectors connect a substantially horizontal bar of a lower facing panel with a substantially horizontal bar of an upper facing panel arranged above the lower facing panel. Thus the connectors serve to connect lower and upper facing panels together as well as to connect the facing to the stabilising elements. The facing may be made up of mesh facing panels which are substantially "L" shaped in vertical cross-section. Typically, the front portion of the "L" will be substantially longer than the rearwardly extending portion, for example at least five times longer and preferably ten times longer. The use of "L" shaped panels rather than the known "C" shaped panels results in more potential deformation along the horizontal joint between the panels, because the rearwardly extending portion at the top of the panels is omitted, thereby reducing the stiffness of the panels. However, the use of a connector having at least two spaced apart attachment portions attached to the facing panel compensates for the loss of stiffness. The "L" shaped panels can be used to form vertical facings and also non-vertical facings, even if the angle between the front portion and rearwardly extending portion of the "L" is 90°, if the connectors are pivotably attached and thus do not have to be at the same orientation as the rearwardly extending portion of the "L". This advantageously permits standardisation of the facing panels for facings of different slopes. Moreover, a particular facing can have portions of different slopes whilst still using the same panels. The connectors are preferably arranged to permit relative vertical movement between the lower and upper facing panels. This can be achieved by the hooks described above, having a vertical play which is greater than the combined thickness of the two horizontal bars. Thus, in a preferred construction method, the horizontal bar of an upper facing panel may be spaced upwardly from the horizontal bar of a lower facing panel by a wedge. This determines the position of the connector and thus the position of the stabilising element in the earth behind the facing. Once the upper facing panel has been backfilled the wedge can be removed and as settlement of the backfill takes place the upper facing panel can move downwardly by the thickness of the wedge before its horizontal bar engages the horizontal bar of the lower facing panel. The lower facing panel is therefore not pushed downwardly by the upper facing panel and thus any tendency for it to bulge forwardly is significantly reduced. In practice, at least two facing panels above each wedge will normally be backfilled before the wedge is removed. The use of "L" shaped facing panels, in preference to "C" shaped panels, advantageously permits relative vertical movement between lower and upper panels. The stabilising elements may take various forms and may for example be in the form of elongate ties connected at their rear ends to dead men anchors in the earth. Such a system operates by retaining a mass of earth between the facing and the dead men anchors. Preferably, however, the stabilising elements are in the form of strips which stabilise the earth by frictional interaction therewith. It is preferred for the rear attachment portions of the connectors to extend laterally. The stabilising strips may for example be attached to the connectors by ties which loop round the laterally extending rear attachment portions. This arrangement is useful if the strips are metal strips, since the forward end of the strips can be secured to the ties by a vertical bolt. Alternatively, the stabilising strips may be attached to the connectors by looping round the laterally extending rear attachment portions, whereby each strip has first and second portions which extend rearwardly from its respective connector. Such an arrangement may be useful if the strips are geosynthetic strips which are generally quite flexible and capable of forming a loop. In order that the turn at the loop is not too tight, a tube of larger diameter may be provided round the laterally extending rear attachment portion of the connectors. Certain preferred embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a front elevation of a first type of mesh facing panel with three connectors, for use in a vertical facing; Figure 2 a plan view of one of the connectors attached to the facing panel and to a stabilising strip; Figures 3 and 4 are respective side views of the connector, to an enlarged scale, before and after removal of a wedge; Figure 5 is a front elevation of a second type of mesh facing panel with two connectors, for use in a sloping, non-vertical facing; Figure 6 is a side view of the sloping facing; Figure 7 is a vertical cross-section through an earth structure with a sloping facing; Figures 8 and 9 are vertical cross-sections similar to Figure 7 and showing the structure during certain stages of construction; Figure 10 is a plan view of a connector attached to a geotextile strip; and Figure 11 is a side view of the connector of Figure 10. Referring to Figures 1-4, a mesh facing panel 1 is provided with three connectors 2 each connected to an earth stabilising element in the form of a galvanised steel strip 3. The facing panel 1 has a vertical facing portion 4 and a relatively short rearwardly extending portion 5 at its lower end, so as to be substantially "L" shaped, the portions 4 and 5 being perpendicular to each other. Each connector 2 has two front attachment portions for attachment to the facing panel 1, each in the form of a hook 6, so that the connector consists of a double-hook arrangement. Extending rearwardly from the double-hook are a pair of converging portions 7 which are joined at the rear of the connector by a laterally extending rear attachment portion 8. A hair pin shaped lug 9 passes round the laterally extending portion 8 and is connected to the front end of the stabilising strip 3 by a vertical bolt 10. The strip has an integrally thickened portion 11, as described in British Patent Application No. 2177140, and a series of ribs 12 for improving frictional interaction with the surrounding earth, as described in British Patent No. 1563317. Referring to Figures 3 and 4, each hook 6 passes round a lower horizontal bar 13 of an upper facing panel and round an upper horizonal bar 14 of a lower facing panel. At the time of construction, a temporary wood wedge 15 is placed at the attachment point between the bars 13 and 14. After the upper panel, and the next upper panel above that, have been backfilled with earth the wedge is removed, so that as the backfill settles and moves the stabilising strip 3 together with the connector 2 downwardly, the upper panel is able to move downwardly from the position shown in Figure 3 to that shown in Figure 4. Thus the upper panel can move downwardly by the thickness of the wedge, which may be 4% of the height of the panel, before it starts to push downwardly on the lower panel, thereby reducing the tendency for the lower panel to bulge forwardly. Thus whilst the double hook arrangement helps to reduce forward deflection of the panels as they span horizontally between the connector attachment points, the arrangement also permits relative vertical movement between vertically adjacent panels and this tends to reduce forward deflection of the panels as they span vertically between vertically adjacent connectors. The facing panel 1 shown in Figures 1-4 is formed of electro-welded steel mesh with a nominal height of 0.625m and a width of 3m. The mesh bars are spaced both vertically and horizontally by 100mm. The vertical bars have a diameter of 10mm and the horizontal bars have a diameter of 8mm apart from the top bar and the last two bottom bars (one in the facing portion 4 and the other in the rearwardly extending portion 5) which have a diameter of 14mm for added stiffness. The use of a lighter facing panel is possible, being more cost effective and appropriate for low height and/or temporary structures. The facing panels have the same geometry but the diameters of all the bars are reduced by for example 2mm. The connectors shown in Figures 1-4 are bent from a 14mm galvanised steel bar. The inner vertical dimension of the hooks 6 is about 60mm. The length of the connector, in the direction extending rearwardly into the earth, is about 0.4m. The centre-to-centre spacing of the connectors is about 1m and their width, which is the spacing between the hooks 6, is about 0.55m. The deflection of the facing panels 1 in the horizontal plane of the connectors and at their centres may typically be 4-6mm. The maximum deflections between the connectors are less. At the edges of the panels the deflection may be about 10mm. These values are acceptable. Figures 5 and 6 show a second type of mesh facing panel 2 for use in a sloping, non-vertical facing (60° to the horizontal). The main difference from the first type of facing panel arrangement is that only two connectors 2 are used, each having a width of about 0.85m and being located at a centre-to-centre spacing of about 1.7m for a 3m wide panel. In addition, the panel is of greater height, having a nominal vertical height of 0.715m. The deflection of the facing panels in the horizontal plane of the connectors and at their centres may typically be about 6mm, with a maximum deflection of about 3mm between the connectors. At the edges of the panels the deflections may be negative (rearward), e.g. about 4mm. It will be noted that the rearwardly extending portions 5 of the facing panels of Figures 5 and 6 are perpendicular to the facing portions 4, as in the case of the vertical facing. This is possible because the connection between each connector 2 and two vertically adjacent panels permits pivoting of the upper facing panel to the desired angle. Thus facing panels having a right angle between their facing portion 4 and rearwardly extending portion 5 can be used to form facings of various slopes, even permitting a change in the slope in the same structure. This is advantageous in that it enables standardisation of the facing panels. As apparent from Figures 4 and 6, at the horizontal joint between vertically adjacent panels the upright bars are simply juxtaposed. There is no recess and the hooks 6 of the connectors are very discreet, the major part of the connectors being behind the facing. The vertical joint between adjacent panels in the same course is also a simple juxtaposition. The earth structure shown in Figure 7 has a first region 20 of soil suitable for plant growth, and a second region 21 of structural backfill. The two regions are separated by geotextile sheets 22. The earth in the first region 20 may be a fine soil such as a silty sand that provides a certain water retention capacity. It can be top soil if the humus content is low and if compacting can be sufficiently achieved. The earth in the second region 21 will tend to be a coarser material with good drainage properties and less aggressive to the stabilising elements. A jute backing or "Enkamat" (trade mark) or the like (not shown in the drawings) is normally placed immediately behind the mesh facing panels to retain fine soil particles until vegetation is established. The construction of the earth structure will now be described with reference to Figures 7, 8 and 9. Posts 23 are driven into the foundation to provide alignment of a first course 31 of facing panels 1. A facing panel of the first course 31 with connectors 2 and stabilising strips 3 is placed and a first geotextile sheet 22 is laid on the soil behind the facing panel. A first layer A of structural backfill is placed on the stabilising strips 3. The geotextile sheet 22 is laid back along layer A and a layer B of top soil is placed between the panel 1 and the geotextile sheet 22. The geotextile sheet 22 is hung on the facing panel 1, as seen in Figure 8, and a second layer C of structural backfill is placed on the first layer A. The geotextile sheet 22 is removed from the facing panel 1 and passed back over layer C. A facing panel 1 of a second course 32 is placed on the facing panel of the first course, along with its connectors 2 and stabilising strips 3. The facing panel 1 is positioned vertically with a wedge 15 between lower horizontal bar 13 and upper horizontal bar 14 of the panel below. It is held in place by a temporary stay 24. The stabilising strips 3 are adjusted as shown by arrow D in Figure 9 to obtain the correct positioning of the facing panel. A second geotextile sheet 22 is laid on the strips 3 and a small volume E of backfill is placed on the strips to fix them in position. The geotextile sheet is folded back to leave a gap behind the facing panels 1 which are then backfilled with top soil layer F. The top soil in layers B and F is carefully compacted. The geotextile sheet 22 in the second course is hung on the facing panel of the second course, as shown in Figure 9, and a backfill layer G is placed on the second course of stabilising strips 3, in a similar manner to the placement of layer A described above. The stay 24 is removed and the panel 1 of the second course 32 is tilted back to the correct orientation and backfilled with top soil layer H, equivalent to layer B described above. The process is continued with further courses of facing panels. In the last course, which in this case is the third course 33, the connectors 2 are hooked to a horizontal bar below the top of the panel 1, so that it can be buried and the tops of the upright bars of the panel can be bent rearwardly and downwardly for safety. The wood wedges 15 are then removed to enable the facing panels to move downwardly as the backfill settles, without significant bulging. Figures 10 and 11 show an embodiment in which geosynthetic strips 40 are used to stabilise the earth. The arrangement is generally similar to the previously described embodiments, except that a tube 41 is placed on a bar before it is bent to the correct shape to form the connector 2. The stabilising strip 40 loops round the tube 41 so as to have upper and lower portions which diverge as they extend rearwardly into the earth behind the facing.

Claims: A stabilised earth structure comprising a plurality of elongate stabilising elements (3;40) in an earth mass behind a facing (4), the earth mass comprising a first region of earth of a first type (20) adjacent to the facing, and a second region of earth of a second type (21) behind the first region, characterised in that the first region of earth comprises soil suitable for plant growth and in that the earth structure further comprises a plurality of connectors (2) behind the facing and connecting it to the stabilising elements, the connectors being located in said first region of earth and the stabilising elements being located in said second region of earth, whereby exposure of the stabilising elements to the first type of earth in the first region is substantially avoided. An earth structure as claimed in claim 1, wherein said first and second regions are separated by geosynthetic material (22). An earth structure as claimed in claim 2, wherein said geosynthetic material (22) is non-woven. An earth structure as claimed in any preceding claim, wherein said connectors (2) are provided separately of the facing (4) and extend therebehind to connect the facing to the earth stabilising elements (3; 40), each connector having a rear attachment portion (8) attached to a respective earth stabilising element, and having at least two spaced apart front attachment portions (6) attached to the facing. An earth structure as claimed in any preceding claim, wherein said earth stabilising elements (3;40) are substantially horizontal. An earth structure as claimed in any preceding claim, wherein said earth mass comprises a plurality of layers (A,B,C,F,G,H) which have been successively placed on to respective layers of earth stabilising elements (3;40). An earth structure as claimed in any preceding claim, wherein said stabilising elements are stabilising strips (40) and are attached to said connectors by being looped around a laterally extending bar (41). An earth structure as claimed in claim 7, wherein said stabilising elements (40) are made of a geosynthetic material.

Inventors: JAILLOUX JEAN-MARC, SEGRESTIN PIERRE, JAILLOUX, JEAN-MARC, SEGRESTIN, PIERRE
Applicants: ILE DES BREVETS HENRI VIDAL SO, SOCIETE CIVILE DES BREVETS HENRI VIDAL
Application year: 1994
Main classification: E02D29
IPC codes: E02D29